In general, a color cathode ray tube has a substantially rectangular panel including an effective portion having a curved inner surface. A phosphor screen consisting of a three-color phosphor layer is provided in the effective portion of the panel. The color cathode ray tube is provided with a mask body and a shadow mask provided in the periphery of the mask body. The mask body is arranged apart from the phosphor screen by a predetermined distance or so-called q-value and has an effective surface in form of a curved surface, which is opposed to the phosphor screen and having a number of electron beam apertures formed therein. In a color cathode ray tube having a structure as described above, three electron beams emitted from an electron gun assembly provided in a neck of a funnel are deflected by a deflector and are subjected to a selection by the shadow mask such that three electron beams correctly enter into the three-color phosphor layer. The phosphor screen is thus scanned with the electron beam and a color image is displayed on the phosphor screen.
In the field of color cathode ray tubes having a structure as described above, it is a main trend to adopt an inline type color cathode ray tube using three-guns as an electron gun assembly, which emits three electron beams arranged in line to pass through one same plane.
In general, only 1/3 or less of electron beams emitted from the electron gun pass through electron beam apertures of a shadow mask and enter into the phosphor screen while the rest of the electron beams collide into the shadow mask. Therefore, the shadow mask is heated to 80.degree. C. or more in several cases. If the shadow mask is thus heated, and particularly, if the mask body is made of a rolled plate having a thickness of about 0.1 to 0.3 mm and a high thermal expansion coefficient and if a mask frame installed at a peripheral portion of the mask body is made of a rolled plated having a thickness of about 1 mm, so-called doming is caused and the thin mask body 1 made of a thin rolled plate expands toward a phosphor screen 2, as shown in FIG. 1, and the distance (or q-value) between the inner surface of an effective portion 4 of a panel 3 and an effective surface of the mask body 1 exceeds a tolerable range, thereby causing deterioration of color purity.
In the doming of the shadow mask 5, two types of doming are caused, one being doming in which the center portion of the mask 1 expands toward the phosphor screen 22 because the mask body 1 is mainly heated and the heat of the peripheral portion of the mask body 1 transfers to the mask frame 6 having a large heat capacity, and the other being localized doming caused by collision of a high-density electron beam 7 for displaying a localized high luminance image as indicated by a one-dot chain line. In particular, deterioration of color purity due to the localized doming easily occurs in the vicinity of a position on the long axis direction of the mask body 1, which is distant from the center of the effective surface of the mask body 1, by about 1/3 of the long axis.
As described above, in a color cathode ray tube, a shadow mask is heated due to collisions of electron beams emitted from an electron gun to cause doming in which a mask body having a small thickness expands toward the phosphor screen, so that the distance between the inner surface of an effective portion of the panel and an effective surface of the mask body exceeds a tolerable range, thereby causing deterioration of color purity. As for the doming, two types of doming are caused, one being doming in which the center portion of the mask expands toward the phosphor screen because the mask body is mainly heated and the heat of the peripheral portion of the mask body is transferred to a mask frame having a large heat capacity, and the other being doming which is caused due to collisions of high-density electron beams for displaying a localized high-luminance image. In particular, there is a problem that deterioration of color purity due to localized doming easily occurs in the vicinity of a position on the long axis of the mask body, which is distant from the center of the effective surface of the mask body by 1/3 of the width of the mask body in the long axis direction.